1. Field of the Invention
The present invention relates to material for forming contact members of a control switch and a control switch using same. More specifically, the invention relates to material for forming contact members of a control switch which has a small coefficient of dynamic friction, good resistance to heat, oxidation and corrosion, and good solderability, and a control switch formed of that material, thereby showing good resistance to adhesive wear and environmental influences.
2. Prior Art
A slide switch, a lever switch, a push switch, a tactile push switch, a dip switch and the like are known generally as a control switch. Each of those switches has a fixed contact member integrally composed of contact pieces and soldering terminals.
A tactile push switch, which belongs in those control switches, is shown in FIGS. 1 and 2. FIG. 2 is a sectional view taken along line II--II in FIG. 1.
As seen in FIGS. 1 and 2, the tactile push switch has a combination of a fixed contact member 1 and a movable contact member 2. The movable contact member is housed, for example, in a resin case 3 with a cover 4. A stem key is mounted thereon.
The fixed contact member 1 is integrally composed of soldering terminals 1a and contact pieces 1b. Conventionally, the fixed contact member is made of material having such formation that an Ag or Ag alloy layer having a thickness of approximately 0.5 to 20 .mu.m is formed on a surface of a conductive base such as a brass base.
The movable contact member 2 is made of material having such formation that an Ag or Ag alloy layer is formed on a surface of a conductive base having a good spring property such as a phosphor bronze base.
The contact members are formed of material having an Ag or Ag alloy layer formed on the surface thereof, since Ag or Ag alloy has good resistance to corrosion and good solderability in addition to good electric characteristics.
In the aforementioned tactile push switch, the soldering terminals 1a of the fixed contact member 1 is constantly exposed to air. Therefore, for example, when the tactile push switch is stored, Ag at the surface of the soldering terminal 1a is sulfidized and/or chloridized due to S and/or Cl constituent in the air and solderability of the soldering terminal 1a is lowered, so that soldering of the tactile push switch becomes difficult. In order to solve those problems, countermeasures are taken such as applying a rust preventive to the soldering terminal 1a, or plating the soldering terminal 1a with solder.
However, even when the rust preventive is applied, the effect of the rust preventive is not sufficient in a bad environment, and solder plating is not industrially practical since it raises the production cost of switches. In the case of the movable contact member 2 which is housed in the case 3, problems caused by sulfidization or the like of its surface is less liable to occur. However, the surface containing Ag of the movable contact member 2 is brought into sliding contact with the surface containing Ag of the contact piece 1b of the fixed contact member 1. Therefore, with the passage of time, adhesive wear is generated, so that contact resistance is increased, and a force required for operating the switching action is increased. Thus, the function of the switch is deteriorated.
In order to prevent the aforementioned adhesive wear, countermeasures are taken such as applying contact oil to the surface of the contact piece of the fixed contact member and the surface of the movable contact member, or reducing contact pressure between those contact members. However, none of those countermeasures can prevent an increase in contact resistance and provide sufficient effects.
As described above, in the fixed contact member of control switch, which is integrally composed of the soldering terminals and the contact pieces, the soldering terminals need to have good resistance to environment and good solderability while the contact pieces need to be made of material which does not generate adhesive wear by contact with the counterpart member (the movable contact member) and does not generate an increase in contact resistance.